This project will investigate the mechanism of the active transport system, or pump, for sodium and potassium ions across animal cell membranes. Ion translocation is coupled to phosphorylation of sodium, potassium transport adenosine triphosphatase, (Na,K) ATPase, from ATP with release of ADP and inorganic phosphate. The phosphate group of the phosphoenzyme is a most sensitive and available reporter group of the reactive state of the enzyme. The project will investigate the transient kinetics of its formation and breakdown under the influence of ligands of the enzyme, such as ATP, ADP, Pi, Na ion, K ion, Mg2 ion. The specific inhibitor ouabain, and their congeners. Phosphorylation will be estimated from (32P, ATP, 32P) or both (in different samples) The approach will be to isolate partial reactions and reactive states of the enzyme bounded by slow steps in the reaction pathway. Preference will be given to studies of interactions of ligand binding at equilibrium. Under conditions where circumstances permit, studies of equilibrium binding of the cations will be attempted. The enzyme will be obtained principally in membrane preparations from kidney, or other rich sources. The suitability of various methods of preparation will be tested.